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Geoplanidae is a family of known commonly as land planarians or land flatworms.
These flatworms are mainly predators of other invertebrates, which they hunt, attack and capture using physical force and the adhesive and digestive properties of their mucus. They lack water-retaining mechanisms and are therefore very sensitive to humidity variations of their environment.
Because of their strict ecological requirements, some species have been proposed as indicators of the conservation state of their habitats. They are generally animals with low vagility (dispersal ability) and with very specific habitat requirements, so they can be also used to accurately determine the distribution of biogeographic realms. Today the fauna of these animals is being studied to select conservation priorities in the Atlantic rainforest in Brazil.
At the other extreme, one species in this family, Platydemus manokwari has become an invasive species in both disturbed and wild habitats in the Pacific Islands, and has damaged the endemic land snail fauna. This species has been found in Europe (France) in 2013 for the first time, and in 2015 in New Caledonia, Wallis and Futuna Islands, Singapore, Solomon Islands, Puerto Rico (first record in the Caribbean), and Florida, USA.
The most distinguishing feature that characterizes land planarians is the presence of a creeping sole, a highly ciliated region on the ventral epidermis that helps them to creep over the substrate. The creeping sole may be wide and flat, occupying most of the ventral surface, or narrow and pronounced, being easily distinguished from the rest of the ventral surface.
Contrary to aquatic planarians, land planarians do not have a distinct brain, i.e., there is no clear frontal cluster of nerve cells other than the ventral nerve plate.
Although there are over 830 known species of Geoplanidae in the world, the diversity of land planarians is still poorly known. The highest diversity occurs in tropical forests in South America, Asia and Australia. Europe and North America are relatively species-poor, while the diversity of Africa may be as high as that of other tropical regions, but is highly understudied.
During unfavorable conditions, such as dry seasons, land planarians tend to seek shelter by burrowing in the soil or building a cyst composed of soil particles united by mucus.
Some species are well adapted to human-disturbed environments and many of those have been introduced in areas outside of their native range. In some localities, such as the United Kingdom, the number of introduced land planarian species greatly surpass the number of described native species.
All planarians feed through a muscular and eversible pharynx located slightly posteriorly to the middle of the body length and opening through a ventral mouth. The pharynx is an extensible tube-like organ bearing a complex muscular coat. It specializes as a penetration organ for those planarians that feed on arthropods; or as a grasping organ for those planarians that feed on other soft bodied invertebrates such as earthworms. All geoplanidae pharynxes are equipped with glandular secretions that externally digest and dissolve their prey.
As part of the soil ecosystem, land planarians feed mainly on other invertebrates, such as , , , , , woodlice, , and . Some may even feed on other land planarians.
Some species of land planarians have become invasive pest species. The New Zealand flatworm Arthurdendyus triangulatus and the flatworm Australoplana sanguinea alba have been introduced in the British Isles and are considered to be pest species because they prey upon earthworms and thus may negatively affect soil structure and fertility. Another species, Platydemus manokwari, has been used as an agent of biological pest control of the introduced giant African snail Achatina fulica in Hawaii, the Maldives, Irian Jaya, and Guam, but has become an even worse pest and today threatens several native snail populations in the Pacific.
Some land planarians show hunting behaviour, using chemical signals to detect their prey. Most land planarians have chemical sensory organs in the anterior part of the body, such as sensory pits and epidermal folds which serve as chemical radars for detecting their food. The mucus trails from the slime of slugs, snails and other planarians orient planarians towards their prey. Different species use different techniques for capturing and immobilizing their prey, such as entrapment with sticky mucus and immobilization by physical force.
Cannibalism has been observed in land planarians.
The following phylogenetic supertree after Sluys et al., 2009 presents the current classification of planarians:
In the former suborder Terricola, land planarians were separated into three families according to morphological features:
Recent phylogenetic analyses, however, revealed that Rhynchodeminae and Microplaninae are not closely related and that Caenoplaninae is closer to Rhynchodeminae than to Geoplaninae. The current classification of land planarian subfamilies is shown in the following phylogenetic tree after Álvarez-Presas et al., 2008. The old subfamilies Caenoplaninae and Pelmatoplaninae have been included as tribes Caenoplanini and Pelmatoplanini inside Rhynchodeminae. Note that Spathula and Romankenkius belong to the family Dugesiidae. Their relocation inside Geoplanidae needs further investigation.
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